This invention relates to a process for the separation of a gaseous mixture, particularly air.
U.K. patent specification Nos. 1,480,866 and 1,541,767, and German Offenlegungsschrift No. 2 702 784 all relate to processes for the separation of nitrogen from air employing an adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its more rapid adsorption of oxygen than of nitrogen. The adsorbent is typically molecular sieve carbon, for example, as described in U.K. patent specification Nos. 1,364,674 and 1,424,501. In operation, a bed of the adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed, most of the oxygen and a proportion of the nitrogen and substantially all of the carbon dioxide and water vapour in the feed are adsorbed, and a nitrogen-rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.
In practice, two adsorbent beds are employed and operated on similar cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is on its adsorption step, the other bed is on it desorption step, and vice versa. Moreover, it is usual to equalise the pressures in the two beds between each step by connecting the two bed inlets together and connecting the two bed outlets together. With these connections made the gas within the void spaces of the bed which has just completed its adsorption step is drawn into the bed which has just completed its desorption step by virtue of the pressure difference which exists between the beds at that stage, and this is found to be beneficial in maximising the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.
In order to connect together the two bed inlets and the two bed outlets as aforementioned stop-valves are opened in passages connecting together the inlets and the outlets respectively. Since, in practice, the initial pressure difference between the two beds when the valves are first opened is relatively large (typically in the order of several bars) there is a tendency for the upward passage of gas through the outlet of the bed at higher pressure to lift or fluidise some of the adsorbent in the bed and thereby cause it to strike other particles and walls of the vessel containing that bed. We have found that repeated lifting of the bed in such a manner reduces the efficiency of the adsorbent.
In the process described in U.K. Patent specification No. 1,480,866 product is taken from the outlet of the bed that is for the time being adsorbing. There is no means described for setting the product flow rate or controlling the pressure of the product. Thus, the product nitrogen-rich gas is obtained at a varying flow rate and pressure. In U.K. specification No. 1,541,767 the gas leaving the bed that for the time being is adsorbing is passed through a throttle valve so as to give an increasing product flow rate as the bed pressure rises. In German specification No. 2 702 784 no means for setting the product flow rate is described.